Redox-Driven Chelation and Kinetic Separation of Select Rare Earths Using a Tripodal Nitroxide Proligand

Abstract

Separation of the rare-earth (RE) elements (Sc, Y, La–Lu) is challenging because of their similar chemical properties, but is necessary for their applications in renewable energy and electronic device technologies. The development of separation processes driven by kinetic factors represents a new area for this field. Herein, we disclose a novel method of separating select rare earths by reacting RE cyclopentadienides with the triradical species tris­(2-tert-butylnitroxyl)­benzyl­amine (1). The key proligand 1 was characterized using a variety of techniques including X-ray crystallography, magnetometry, and EPR spectroscopy. When applied to an equimolar mixture of La:Y cyclopentadienide complexes, different rates of chelation of these organometallic precursors by 1 were observed, affording a separation factor of 26 under the reported conditions.

Understanding Molecular Factors That Determine Performance in the Rare Earth (TriNOx) Separations System

“…Several alternative separations techniques have been demonstrated in recent studies including processes based upon selective crystallization and precipitation, − photochemical reduction, , or chemical kinetics, , rather than liquid–liquid extraction. , Chromatographic separations using functionalized materials − and bioinspired separations processes that rely upon bioadsorption , or mineralization techniques have also been reported recently. There is general interest in the development of a system in which modification of the local coordination sphere of the RE cations leads to the optimization of separation efficiencies for a targeted RE mixture.…”

“…Previously, the RE­(TriNOx) separation system, in which binary LRE/HRE mixtures were separated with a single leaching step in benzene or toluene solutions, was caused by differences in solubilities of monomeric/dimeric species formed in situ . , It was determined that LRE complexes more-readily formed a benzene or toluene-soluble dimeric species, [RE­(TriNOx)] 2 , while the smaller, HRE cations did not self-associate in these solutions and remained largely insoluble in aromatic organic solvents. It was demonstrated that the performance of the RE­(TriNOx) separation system could be influenced by using the redox properties of the ligand , or by changing the apical ligand (Figure ). …”

“…Previous and current modifications to the RE­(TriNOx) separations system by the Schelter group (references ,, and − ).…”

“…24 System design, steric-or electronic-tuning, and serendipity have been identified as key factors to improve the performances of solution-based or immobilized extractants. 24,25 Several alternative separations techniques have been demonstrated in recent studies including processes based upon selective crystallization and precipitation, 26−30 photochemical reduction, 7,31 or chemical kinetics, 32,33 rather than liquid−liquid extraction. 34,35 Chromatographic separations using functionalized materials 36−38 and bioinspired separations processes that rely upon bioadsorption 39,40 or mineralization 41 techniques have also been reported recently.…”

Halide anion discrimination by a tripodal hydroxylamine ligand in gas and condensed phases

“…In that context, functional groups such as (thio)urea, amide, pyrrole, or imidazole have attracted considerable attention due to their donor/acceptor properties and geometrical features. 15,[22][23] Given our interest in hydroxylamine ligands (R 1 R 2 NOH), [25][26][27][28][29][30][31] and recognizing their potential for anion binding by means of vicinal H-bond acceptors associated with mildly acidic protons, we initiated studies on the propensity of a tripodal receptor (H3TriNOx) for anion capture (Scheme 1). As hydroxylamine moities have not been examined in this context previously, it was of interest to interrogate their interactions with halides under a range of conditions, including condensed and gas phases, to determine fundamental thermodynamic trends.…”